Intravenous infusion of angiotensin II (AII) alters the baroreceptor reflex such that heart rate and plasma concentrations of vasopressin and ACTH are elevated above normal at each level of arterial blood pressure. To determine whether AII increases the levels of these hormones and heart rate by decreasing the tonic inhibitory effect of input from arterial and low pressure stretch receptors, the effect of abroreceptor denervation on the vasopressin, ACTH and cardiac response to AII will be studied. The role of the area postrema in the responses will be investigated by determining the effect of intravertebral infusion of AII or lesions of the area postrema on function of the baroreceptor reflex. Baroreflex function will be assessed by examining the relationship between blood pressure and each of three parameters: heart rate and systemic concentrations of vasopressin and ACTH. Blood pressure will be varied either by infusion of nitroprusside and phenylephrine, or by hemorrhage. Vasopressin infusion also appears to affect baroreflex function by enhancing the strength of the reflex. To determine whether the area postrema mediates this action, the effect of vasopressin infusion on baroreflex control of ACTH secretion in dogs with lesions of this structure will be studied. Whether vasopressin infusion enhances reflex release of ACTH secretion will also be investigated. Finally, the role of endogenous AII and vasopressin in regulation of the baroreceptor reflex during hemorrhage and water deprivation will be examined by infusion of blockers of the renin-angiotensin system or a vasopressin antagonist.